1. Field of the Invention
The present invention relates to a regeneration control method for an exhaust gas purification system and an exhaust gas purification system equipped with an exhaust gas purification apparatus that purifies the exhaust gas in an exhaust passage in an internal combustion engine.
2. Background Art
Exhaust gas control for automobiles has increased in strictness, and a situation is coming in which it cannot be pursued only by the technical development of the engine side. It has become necessary and indispensable to purify the exhaust gas with a post process apparatus. Because of that, various researches and proposals have been made on a NOx catalyst to reduce and remove NOx (a nitrogen oxide) from the exhaust gas of an internal combustion engine such as a diesel engine and a part of gasoline engine and various combustion apparatuses, and on a diesel particulate filter apparatus (below, a DPF apparatus) that removes particle-shaped substances (Particulate Matter: below, PM) in the exhaust gas of these.
In these researches and proposals, a NOx occlusion reduction type catalyst, a NOx direct reduction type catalyst, etc. are proposed as the NOx purification catalyst.
A NOx occlusion reduction type catalyst apparatus carrying a NOx occlusion reduction type catalyst is configured by carrying a noble metal catalyst having an oxidation function and a NOx occlusion material having a NOx occlusion function such as an alkaline metal, and with these, two functions of NOx occlusion and NOx discharging and purification are exhibited depending on the oxygen concentration in the exhaust gas.
That is, in the case that the air fuel ratio of the exhaust gas flowing into the NOx occlusion reduction type catalyst apparatus has a lean state, nitrogen monoxide in the exhaust gas becomes nitrogen dioxide by being oxidized by the noble metal catalysts, and this nitrogen dioxide is occluded by a NOx occlusion material as a nitrate. On the other hand, in the case of the air fuel ratio of the exhaust gas having a rich state, NO2 is discharged by a nitrate being decomposed from the NOx occlusion material, and this NO2 is reduced to nitrogen by non-combusted hydrocarbon and carbon monoxide in the exhaust gas due to a catalyst action of the noble metal catalysts. Because of that, when the NOx occlusion ability of the NOx occlusion material comes closer to saturation, the NOx recovery control for recovering NOx occlusion ability is performed to make the air fuel ratio in the exhaust gas a rich state.
Further, a NOx direct reduction type catalyst apparatus carrying a NOx direct reduction type catalyst is configured by carrying a metal such as rhodium and palladium, that is a catalyst component, in a carrier such as a β type zeorite, and directly reduces NOx. Then, oxygen is absorbed into the metal that is an active substance of the catalyst at this reduction, and NOx reduction performance deteriorates. Therefore, in order to recover the NOx reduction performance, the air fuel ratio of the exhaust gas is made to be a rich state, a NOx regeneration control action for recovering the NOx reduction performance is performed, and activation is performed by regenerating the active substance of the catalyst.
In these exhaust gas purification systems equipped with a NOx purification catalyst apparatus, the NOx regeneration control that performs this air fuel ratio rich control is normally controlled to start automatically when a certain condition of NOx occlusion amount, NOx purification rate, lean continuing time, etc. reaches a threshold set in advance.
Then, combustion in a normal operation condition that is not under NOx regeneration control is in an air fuel ratio lean state of excess oxygen condition. Therefore, in order to obtain an air fuel ratio rich state with this NOx regeneration control, it is necessary to decrease the intake amount, increase the fuel amount, or perform both, that is to perform any of these air fuel ratio rich controls for regenerating NOx.
In the intake rich control that decreases the intake amount, there is a method such as increasing the EGR amount, exhaust throttle, and intake throttle. Further, in the fuel rich control that increases the fuel amount, there is a method such as post injection in injection in a cylinder (in a cylinder) and direct injection in an exhaust pipe in which fuel such as light oil is injected directly into the exhaust pipe and supplies a reducing agent such as HC and CO into the exhaust gas flowing into a NOx purification catalyst apparatus.
This direct injection in the exhaust pipe has an advantage that it does not influence the engine operation condition compared with the post injection. An exhaust purification apparatus of an internal combustion engine and an exhaust purification system of an internal combustion engine using this direct injection in the exhaust pipe are proposed in Japanese Patent Application Kokai Publication No. 2003-269155 and Japanese Patent Application Kokai Publication No. 2004-346798 for example.
However, in the direct injection in the exhaust pipe in which the air fuel ratio lean state is switched to an air fuel ratio rich state during NOx regenerating control action, in the case of performing a fixed amount injection, an objective supply amount is not reached instantly due to the characteristic of the reducing agent injection apparatus even when the reducing agent is started to be injected from the reducing agent injection apparatus into the exhaust pipe. That is, a delay in response time occurs. Because of that, as shown in FIG. 4, the air fuel ratio condition (air excess rate λ) in the exhaust gas cannot be made to be a rich state instantly and it becomes a gentle transition as shown in FIG. 4A.
As a result, because oxygen exists in the exhaust gas at the initial stage of the regeneration control, the supplied reducing agent is oxidized by a catalytic action of the catalyst metal of the NOx occlusion reduction type catalyst and consumed. As a result, NOx discharged from the NOx occlusion reduction type catalyst cannot be reduced sufficiently.
Then, because the discharge amount of NOx during NOx regeneration is large at the beginning of the regeneration, a large amount of NOx is discharged (slipped) in the downstream side of the NOx occlusion reduction type catalyst apparatus, even if it is temporary, as shown in FIG. 4B, and there is a problem that the NOx purification performance deteriorates. Further, because the consumption amount of the supplied reducing agent becomes less, heat generation due to the oxidation of the reducing agent becomes insufficient, the increase of temperature of the NOx occlusion reduction type catalyst becomes slow, and a problem also occurs in which activation of the catalyst delays.
Further, after that, the reducing agent amount becomes large and the condition becomes deeply rich as shown in FIG. 4C, and the reducing agent corresponding to NOx that is not reduced is supplied into the exhaust gas after the discharge of NOx is completed as well. Because of that, as shown in FIG. 4D, the reducing agent flows out to the downstream side of the NOx occlusion reduction type catalyst apparatus, and a problem of HC and CO slip occurs. Further, because the reducing agent that does not contribute to the reduction of NOx is supplied, deterioration of fuel economy is invited.
This problem of the reducing agent supply occurs not only in an exhaust gas purification system of an independent NOx occlusion reduction type catalyst apparatus, but also in an exhaust gas purification system in which an oxidation catalyst is arranged in the upstream side of the NOx occlusion reduction type catalyst apparatus, etc.
The present inventors obtained knowledge as follows as a result of performing experiments, simulated calculation, etc. based on these results.
During NOx regeneration, as shown in FIG. 3, the concentration of the reducing agent in the exhaust gas flowing into the NOx occlusion reduction type catalyst apparatus is rapidly made to be a high concentration (E, F) of about 0.70 to 0.90 in excess air ratio conversion at the beginning of the regeneration, and after that, it is made to a low concentration (G) of about 0.80 to 1.00 in excess air ratio conversion. With this, the peak of the NOx concentration in a downstream side of the NOx occlusion reduction type catalyst apparatus at the beginning of the regeneration can be made small as shown as H in FIG. 3. Further, a peak (J) of the HC concentration and the CO concentration in the downstream side of the NOx occlusion reduction type catalyst apparatus right before completion can be eliminated even if there is a small peak (I) at the beginning of the regeneration.
On the other hand, an exhaust gas purification system equipped with a catalyst carrying DPF apparatus (CSF) in which a NOx occlusion reduction type catalyst is carried in a DPF apparatus that collects PM in the exhaust gas and a NOx occlusion reduction type catalyst apparatus in its downstream side is proposed.
In this exhaust gas purification system, in order to avoid excessive clogging up of the DPF, a PM regeneration control action is performed when the sediment amount of the collected PM exceeds a prescribed judgment value. In this PM regeneration control, in the case that the temperature of the exhaust gas flowing into the catalyst carrying DPF apparatus is low, the carried catalyst is activated by increasing the temperature and a reducing agent such as a fuel is supplied into the exhaust gas from a reducing agent injection apparatus. The PM is removed by combustion by oxidizing this reducing agent with a catalytic reaction of the carried catalysts and by increasing the temperature of the catalyst carrying DPF apparatus to an initial temperature of PM combustion or more with this oxidization heat.
In order to perform re-combustion of PM in the current system, first the temperature of the supplied exhaust gas is increased to an activation temperature of the oxidization catalysts arranged in a front step (the upstream side) with an after injection (a post injection), etc., and then the temperature of the exhaust gas is increased to the PM re-combustion temperature of about 600° C. With this, PM is combusted.
In this case, there is a problem that deterioration of fuel economy is large because the temperature of the entire amount of the exhaust gas supplied to the catalyst is increased to 600° C. Further, because the PM combustion is local combustion, in the case that the average temperature of the exhaust gas is as high as 600° C., the local temperature reaches to about 800° C., and there is a problem that early deterioration of the carried catalyst is caused.
Then, because frequent PM regeneration as described above causes extreme deterioration of fuel economy, PM regeneration combustion is performed after accumulating a considerable amount of PM in a filter. Because of that, the local temperature becomes higher, and there is a problem that the exhaust pressure increases due to the accumulation of PM and further deterioration of fuel economy is caused.
For this DPF apparatus, the present inventors performed experiments, simulation calculation, etc. As a result, knowledge was obtained in which the said problem can be solved by performing the following PM regeneration when a small amount of PM is accumulated at the initial stage of the PM accumulation without increasing the temperature of the entire amount of the exhaust gas that becomes a cause of the deterioration of fuel economy in the case of performing the PM regeneration control without arranging the oxidization catalyst in the front step of the filter.
In the PM regeneration control, as shown in FIG. 6, the smaller the PM accumulation amount, the larger the PM regeneration speed is. That is, in the PM combustion, the smaller the PM accumulation amount is, the more the combustion speed increases and PM disappears in a short time, and on the contrary, the combustion speed decreases extremely as the PM accumulation amount becomes large, and the combustion becomes difficult. Because of that, the PM regeneration is performed on a small amount of the PM accumulation at the initial stage of the PM accumulation. For example, the PM regeneration control is preferably performed once every 5 to 6 times that the NOx regeneration is performed.
Therefore, in this PM regeneration control, deterioration of the fuel economy is small because the temperature of the entire amount of the exhaust gas is not increased, and deterioration of the fuel economy can be prevented because the PM regeneration is performed at the initial stage where an increase of the exhaust pressure due to the PM accumulation is small.
Then, in this PM regeneration control, fuel is directly added in the exhaust gas, and HC, that is a reducing agent, is supplied in the catalyst carrying DPF apparatus (CSF). In this case, the beginning of the PM regeneration control is made to be a lean state in which the amount of fuel is relatively small as being 1.5 to 5.0 in excess air ratio conversion. After that, it is made to be a rich state of 1.0 to 0.9 in excess air ratio conversion. After that, it is again made to be a lean state in which the amount of fuel is relatively small as being 1.5 to 5.0 in excess air ratio conversion. With this, HC and CO can be purified while preventing discharge of NOx also during the PM regeneration, and quick combustion of PM can be completed without excessively increasing the temperature of the exhaust gas. Therefore, it was found that deterioration of fuel economy and deterioration of the catalyst can be prevented.
When HC is absorbed on the surface of the oxidization catalyst of the catalyst carrying DPF apparatus in this initial lean state, there is sufficient oxygen, and therefore a small amount of the accumulated PM can be ignited and combusted. With this ignition, the temperature is increased locally only on the surface of the oxidization catalyst. With this increase of the temperature, NO2 that has been occluded to the catalyst is discharged. PM is combusted with this NO2, and NO2 becomes NO. Discharge of this discharged NO into the atmosphere is prevented by being occluded by the NOx occlusion reduction type catalyst in the downstream side.
With the rich state in which the fuel is supplied densely after that, the NO occluded to the NOx occlusion reduction type catalyst arranged in the downstream side is discharged, and at the same time, the reduction and purification is performed with the oxidation catalyst. HC and CO that are not used (excess) in this reduction are oxidized and purified or absorbed and purified with a three-dimensional catalyst or the oxidation catalyst in the downstream side of the NOx occlusion reduction type catalyst. Further, NOx (slipped NOx) flowing out in the downstream side of the NOx occlusion reduction type catalyst is also purified at the same time. Then, higher HC and CO purification activity can be obtained by making a lean state in which fuel is relatively small again.
However, in the conventional art, it is not performed that the concentration of the reducing agent in the exhaust gas flowing into the exhaust gas purification apparatus is changed with time so as to correspond to the temporal change of the condition of the exhaust gas purification apparatus during regeneration when the reducing agent is supplied at the regeneration of the exhaust gas purification apparatus. Because of that, it is desired to perform a proper control action.
Patent Document 1: Japanese Patent Application Kokai publication No. 2003-269155
Patent Document 2: Japanese Patent Application Kokai publication No. 2004-346798